This invention relates to bifocal lenses and in particular to bifocal and trifocal contact and intraocular lenses.
Various designs of bifocal contact lenses have been proposed but all require considerable amounts of chair-side time for satisfactory fitting of such lenses. Designs have been proposed in which the viewing area consists of a central circular zone, surrounded by concentric zones of alternating near (reading) and distant vision. U.S. Pat. No. 4,890,913 discloses such a lens. The underlying concept in the above patent is to try to ensure that at every pupil size the amount of light transmitted through the reading and distant vision zones is substantially equal. It has been found that with such lenses when correctly fitted, the wearer is able to concentrate on the clearest image focused on the retina.
While the lenses described in the above U.S. patent have made a useful contribution to the art, and can reduce the amount of fitting time required, there are still some problems to be overcome. Lenses manufactured in accordance with the above patent are most effective when manufactured as hard lenses, or from lens material which have a low water content. If, on the other hand, lenses are made in accordance with the prior patent in a soft, high water content lens material, the zones formed on a posterior lenticular surface tend to be xe2x80x9cironed outxe2x80x9d by the pressure of the upper eyelid.
Another, unrelated problem of some prior art lenses, is that in certain light conditions, reflections are seen from junctions between adjacent zones. This can have the effect of giving the appearance of rings around a light source.
The present invention is, therefore, in one aspect directed to a solution to the above problems and the provision of a lens which can be used both as a contact lens, and as an intraocular lens from a variety of conventional lens materials.
According to the present invention there is provided a contact or intraocular bifocal lens comprising a viewing area which has an extent generally corresponding to the maximum pupil area of the wearer, said viewing area having a central circular refractive zone having a first focal length corresponding to either distance or reading vision, and a plurality of annular, concentric refractive zones which alternate between a second focal length corresponding to the other of reading and distance vision and said first focal length as they extend outwardly from said central zone, wherein the total number of zones is at least 20. Preferably, the central circular zone has a focal length corresponding to distance vision.
The provision of a very large number of alternating reading and distance zones in the viewing area is believed to be responsible for overcoming many of the problems of the prior art lenses. One significant advantage is that by providing a large number of zones, it is much easier to ensure that the relative areas of the distance and reading zones is close to the preferred 50:50 ratio at all pupil sizes. As a consequence, the lenses provide good distance and reading vision in all lighting conditions. The number of zones is at least 20 and is preferably much higher, e.g. at least 30 and preferably at least 50. A typical number of zones may be between 20 and 50, e.g., 25 to 40. There is no theoretical upper limit but there will be a practical upper limit which is determined by the limitations of the equipment or system used to form very large numbers of zones on the surface of a lens. Existing computer controlled lathes should be capable of forming up to about 70xcx9c80 alternate zones on the lenticular surface of a standard contact lens, in which the major viewing area is about 6xcx9c8 mm in diameter.
The alternating zones may be formed on the front surface of the lens, but are preferably formed on the rear surface of the lens. One advantage of forming the zones on the posterior surface of the lens is that any imperfection in the surface is largely compensated by the tear fluid which will fill the space between the undulating profile forming the zones and the cornea.
Preferably, the maximum tear thickness between the anterior lens surface and the cornea is about 0.007 mm and may be as little as 0.003 mm.
In general, the maximum thickness of material removed between every other zone to form the zones is about 10% of the total thickness of the lens.
Another, separate problem arises in fitting bifocal contact lenses. This relates to the observation that eyes suffer from the fact that the combined power of the cornea and the natural crystalline lens increases towards the periphery. As a result, when the pupil dilates in low lighting conditions, a patient with normal distance vision will experience improved distance vision when wearing corrective lenses with a negative power of about 0.5 dioptres.
Another aspect of the present invention is based on the realisation that improved vision with bifocal lenses can be achieved by introducing a progressive power variation in a multi-zonal concentric lens in which the negative power of one or both types of vision zones is increased towards the periphery of the viewing area of the lens. Such progression in negative power should not exceed about 1 dioptre, preferably not more than about 0.75 to 0.8 dioptre and, generally, will be in the range of 0.25 to 0.75 dioptre. A power change of about xe2x88x920.5 dioptre from centre to periphery being preferred.
According to a further aspect of the present invention, therefore, there is provided a bifocal contact lens wherein, at least, the major viewing area is divided into a plurality of near and distance vision zones which are formed as annular zones of different radii from a point in the region of the centre of the lens, each near vision zone being adjacent to a distance vision zone or intermediate vision zone and, wherein at least one of the zones has a power which is more negative than a zone of the same character which is situated closer to the centre of the lens. Lenses in accordance with this aspect of the invention do not necessarily have a larger number of alternating zones as described above. However, in a preferred embodiment, the number of zones is at least 20 and often up to 70 or 80.
Preferably, at least one distance vision zone has a power which is more negative than a distance vision zone situated closer to the centre of the lens. Generally, the distance vision zones have powers which progressively become more negative towards the periphery of the lens viewing area so that the zones towards the periphery have the highest degree of negative power.
In general, there is no further increase in clarity by increasing the negative power by more than about 1 dioptre. Normally, the increase in negative power will be limited to 0.75 to 0.8 dioptre. Usually, optimum benefit is obtained where the negative power increase is in the region of 0.5 to 0.6 dioptre. The maximum additional negative power at the periphery is limited by the onset of reverse spherical aberration.
In the manufacture of lenses in accordance with both aspects of the invention, the zones may be formed by machining the power or base curve of the lens. Preferably, however, the power is produced by machining or moulding the power surface of the lens. In order to provide a smooth transition between zones, it is preferable to make the zones at least partially aspherical, generally by making some or all of the near vision zones aspherical, so that aspherical lens surfaces merge smoothly into spherical lens surfaces with minimum discontinuity at the junction between two zones. Lenses produced in accordance with this concept are described in UK Patent Application No. 2,295,686 (de Carle), and the content of this prior UK specification is specifically incorporated herein.
It may also be advantageous to manufacture lenses in accordance with the invention in such a way that the optical centre of at least the major viewing part of the lens is displaced by a small distance from the geometric centre of the lens. The displacement is such that when the lenses are worn, the lens is offset nasally on the cornea. This concept is described in PCT/WO 98/53360 (de Carle) and in European Patent Application No. 0618474 (Menicon), and the subject-matter of these specifications is specifically incorporated herein.
In accordance with the second aspect of the invention, some or all of the zones may be provided by diffraction as described in U.S. Pat. No. 4,637,697 (Freeman). Lenses in accordance with the first aspect of the invention generally operate by refraction.